Epitaxy is a process commonly used in semiconductor processing to fabricate high quality materials with extremely uniform electrical properties for semiconductor devices. As semiconductor devices have grown smaller, and manufacturing units larger, the need for uniformity across a single manufactured substrate is extreme.
In a typical epitaxy chamber, process gases flow across a substrate from one side of the chamber to the other side, where exhaust gases are removed. The substrate typically rotates during processing to minimize the effect of non-uniformities, but persistent non-uniformities may nonetheless manifest as radial variation.
In a high-growth rate epitaxy chamber, the processing volume is typically very small a close to the susceptor on which the substrate is positioned. A quartz window is disposed over the processing volume to confine the process gases to the processing volume. The dome may be slightly curved at the edges where the dome meets the side of the chamber. As the dome curves toward the side of the chamber, the processing volume compresses, so the flow path for process gases near the edge of the substrate is very small. Consequently, the process gases flow at very high velocity.
The epitaxy chamber usually includes a pre-heat ring or ring assembly that circumnavigates the susceptor. The pre-heat ring assembly typically absorbs heat from heating elements below the susceptor and re-radiates that heat above the pre-heat ring assembly near the edge of the susceptor. The radiated heat increases the temperature of incoming process gases to a processing temperature before the gases reach the susceptor, and the substrate disposed thereon. This ensures reaction of gases with the substrate surface starting at the edge of the substrate.
With the compressed process volume and high gas flow rates near the side of the chamber, gas heating over the pre-heat ring assembly is compromised. Residence time over the pre-heat ring assembly may be insufficient to allow the process gases to above enough heat to activate for epitaxial growth at the edge of the substrate. Thus, growth at the edge of the substrate is reduced, and uniformity suffers.
There is a need for an apparatus to pre-heat process gases for epitaxy in a very high flow-rate situation.